1. Field of the Invention
The present invention relates to a backlight unit and a display device including the same.
2. Description of Related Art
A liquid crystal display device, which is a kind of a display device, includes a liquid crystal display panel for displaying an image. The liquid crystal display panel does not emit light, and hence a backlight unit is placed on a rear surface side of the liquid crystal display panel (side opposite to a display surface side of the liquid crystal display panel) so that the liquid crystal display panel is illuminated by the backlight unit, to thereby enable display operation.
As a light source to be used for the backlight unit, there is known a cold cathode fluorescent lamp formed of a fluorescent tube sealing mercury or xenon therein. However, when the cold cathode fluorescent lamp is employed as the light source for the backlight unit, there has been inconvenience as follows. That is, the cold cathode fluorescent lamp fails to attain a sufficient light-emitting luminance and sufficient life. In particular, the luminance on a low-pressure side is lowered, which makes it difficult to attain well-balanced luminance.
In order to resolve the above-mentioned inconvenience, there is proposed a backlight unit which employs, as the light source, a light emitting diode (LED) instead of the cold cathode fluorescent lamp. Such a backlight unit is disclosed, for example, in JP 2008-84537 A. When the LED is employed as the light source as in the backlight unit thus proposed, a high luminance may be attained with low power consumption. In addition, environmental load may also be reduced.
It should be noted that there are various methods of producing white light by using the LED. For example, one of the methods is to use a phosphor which converts blue (blue-violet) LED light into yellow light, in combination with a blue LED (blue-violet LED). Another one is to use phosphors converting blue (blue-violet) LED light into green light and red light, respectively, in combination with a blue LED (blue-violet LED). There is still another method which uses three kinds of LEDs, namely, a blue LED, a green LED, and a red LED in combination.
The backlight unit placed in the liquid crystal display device generally falls into two types, namely, a direct type and an edge light type.
Their structures are described briefly. A direct type backlight unit has a light source placed immediately below a liquid crystal display panel, and light emitted from the light source illuminates the liquid crystal display panel through optical sheets (a diffusing sheet, a lenticular sheet, a polarizing sheet, and the like).
An edge light type backlight unit, on the other hand, has a light guide plate placed immediately below a liquid crystal display panel, and has a light source opposed to a predetermined side surface of the light guide plate. In illumination operation of the edge light type backlight unit, light emitted from the light source is introduced into the light guide plate from the predetermined side surface of the light guide plate. The light introduced into the light guide plate is repeatedly reflected, exits in a planar manner from a front surface of the light guide plate (the surface facing toward the liquid crystal display panel), and then illuminates the liquid crystal display panel through optical sheets.
Those two types of backlight units have their respective uses. Liquid crystal display devices that are focused on being slim employ edge light type backlight units which are advantageous in reducing thickness.
In an edge light type backlight unit that uses an LED, a plurality of LEDs are mounted on the same printed board to constitute a module, and the LED module is placed to oppose a predetermined side surface, namely, a light incident surface, of a light guide plate.
A concrete structure of the LED module is as illustrated in FIG. 18, and includes at least a plurality of LEDs 101 and a printed board 102 having a mounting surface 102a on which the plurality of LEDs 101 is mounted. The plurality of LEDs 101 are arranged in the longitudinal direction of the printed board 102. Metal wiring patterns 103 arc provided on a surface of the printed board 102 where the mounting surface 102a is located. The plurality of LEDs 101 are bonded (mounted) to the metal wiring patterns 103 and thus electrically connected to one another in series.
The printed board 102 also has fixing portions (for example, notches) 102b formed along one longer side S101 of the printed board 102 and the other longer side S102 thereof, respectively, in order to fix the printed board 102. The printed board 102 is fixed by fastening the fixing portions 102b of the printed board 102 to a chassis (not shown) with screws or the like.
The LED module of FIG. 18 needs to be thin because, as mentioned above, edge light type backlight units are often employed in liquid crystal display devices that are focused on being slim. The width of the printed board 102 in the shorter-side direction is therefore usually set as small as around 10 mm.
The metal wiring patterns 103 are provided on the same surface of the printed board 102 as the mounting surface 102a. The distance between one metal wiring pattern 103 and another metal wiring pattern 103 or the distance from an outer edge of the printed board 102 to one of the metal wiring patterns 103 is called a creepage distance, which needs to be long enough to ensure the insulation of the metal wiring patterns 103. The necessary creepage distance, which varies depending on how high voltage is applied to the metal wiring patterns 103, is approximately 1 mm when the maximum voltage applied to the metal wiring patterns 103 is 200 V to 300 V.
If the LED module of FIG. 18 does not have a sufficient creepage distance between partial metal wiring patterns 103a of the metal wiring patterns 103 and the fixing portions 102b of the printed board 102, the partial metal wiring patterns 103a of the metal wiring patterns 103 need to be displaced toward the longer side S102 of the printed board 102. In other words, the mount point of the LEDs 101 that are to be bonded to the partial metal wiring patterns 103a of the metal wiring patterns 103 (hereinafter, referred to as LEDs 101a) needs to be displaced toward the longer side S102 of the printed board 102 with respect to the mount point of the other LEDs 101 (hereinafter, referred to as LEDs 101b).
This displacement may be made, for example, as illustrated in FIGS. 19 and 20, where the mount point of each LED 101a is off the center of a light incident surface 104a of a light guide plate 104 in the thickness direction of the light guide plate 104, and the mount point of each LED 101b is closer to the center of the light incident surface 104a of the light guide plate 104. In this method, however, one of components of light indicated by arrows L101 to L104, specifically, the component of light corresponding to the arrow L101, misses a region which light emitted from the LED 101a enters (see FIG. 19) out of all regions of the light incident surface 104a of the light guide plate 104. A region which light emitted from the LED 101b enters (see FIG. 20), on the other hand, makes use of all the components of light indicated by the arrows L101 to L104. The arrows illustrated in FIGS. 19 and 20 schematically represent light emitted from each LED 101.
Consequently, the difference increases between the light incidence efficiency in regions of the light incident surface 104a of the light guide plate 104, which light emitted from the LEDs 101a enters, and the light incidence efficiency in regions of the light incident surface 104a, which light emitted from the LEDs 101b enters. This hinders light from uniformly entering all regions of the light incident surface 104a of the light guide plate 104 and results in uneven luminance.
The uneven luminance is prevented by displacing the mount point of the LEDs 101b toward the longer side S102 of the printed board 102 along with the mount point of the LEDs 101a, and thus aligning all of the plurality of LEDs 101 on the same straight line running in the longitudinal direction of the printed board 102. However, displacing the mount point of the LEDs 101b toward the longer side S102 of the printed board 102 makes it difficult to secure a necessary creepage distance from partial metal wiring patterns 103b of the metal wiring patterns 103 (see FIG. 18).
Another possible method of preventing the uneven luminance is to increase the thickness of the light guide plate 104. A drawback of this method is that it presents an obstacle in reducing the thickness of a liquid crystal display device.